Hydrocarbon conversion processes such as catalytic cracking, pyrolysis, hydroprocessing, reforming, and the like can be utilized for producing conversion products comprising molecular hydrogen, saturated hydrocarbon (such as methane, ethane, propane, butane, etc.), and unsaturated hydrocarbon (e.g., ethylene, propylene, etc.). The mixtures generally contain undesirable compounds, such as hydrogen sulfide, carbon dioxide, water, and carbon oxysulfides, such as carbonyl sulfide (“COS”). It is generally desirable to separate ethylene and/or propylene from the conversion product, e.g., for storage and/or further processing, such as polymerization. In order to simplify this separation, at least a portion of the undesirable impurities are typically removed upstream of the ethylene and/or propylene separation. It is also typical to remove from the conversion product one or more of methane, C4+ hydrocarbon, and molecular hydrogen, in order to increase ethylene and/or propylene purity.
For example, PCT Patent Application Publication No. WO2014/078153A1, which is incorporated by reference herein in its entirety, discloses utilizing successive fractionation stages for removing molecular hydrogen, methane, and C4+ hydrocarbon from the conversion product. By exposing the conversion mixture to successively lower temperatures, hydrocarbon present in the conversion process can be separated by fractional distillation in sequential distillation stages, e.g., in one or more cryogenic distillation stages, such as in one or more cold boxes.
At least a vapor stream and a condensed stream are conducted away from the cold box. The vapor stream (a “tail gas”) comprises methane and molecular hydrogen. The condensed stream comprises C2+ hydrocarbon. Fractional distillation can be used for separating from the condensed stream one or more of a (C2+C3) hydrocarbon stream and a C4+ hydrocarbon stream. Ethylene and/or propylene can be separated from the (C2+C3) hydrocarbon stream using conventional methods. It is generally desirable to separate molecular hydrogen from the tail gas, e.g., to increase the tail gas's methane concentration and to make molecular hydrogen available for other uses. It is conventional to use additional cold box stages to do this.
One difficulty encountered in separations carried out at cryogenic temperatures involves the accumulation of at least a portion of the undesirable compounds, such as COS, in the cold box or other low-temperature region of the process. The closeness of the COS and C3 hydrocarbon boiling points leads to another difficulty, namely the presence of COS in condensed hydrocarbon streams containing C3 hydrocarbons and/or mixtures of C2 and C3 hydrocarbons. Since cryogenic separation is cost-intensive, it is also generally desirable to make these stages as small as practical, e.g., to process the minimum amount of material necessary to obtain the desired ethylene and propylene products. For at least these reasons, it is conventional to remove at least a portion of any undesired impurities from the conversion product before the conversion product is exposed to sub-ambient temperatures in the cold box, as disclosed in the WO2014/078153A1 reference.
One way to remove carbon oxysulfides involves the use of a regenerable selective sorbent. When the conversion gas is conducted through a bed of regenerable selective sorbent operated in sorption mode, at least a portion of the carbon oxysulfides in the conversion gas remains in the bed. By selecting appropriate sorbents, bed configurations, and process conditions (temperature, pressure, flow rate, etc.), the concentration of carbon oxysulfides in the conversion product can be lessened to very low level, e.g., ≦1 part per million by mole (1 ppmm), per mole the conversion product. When the selective sorbent bed approaches its maximum capacity for the removal of carbon oxysulfides, the bed conditions are changed from sorption mode to regeneration mode, during which the bed's sorbent is regenerated. Regeneration is typically carried out by flowing tail gas through the bed at an elevated temperature to desorb the carbon oxysulfides, resulting in a tail gas enriched in carbon oxysulfides (“rich tail gas”). The process continues by changing the bed conditions to sorption mode, after regeneration is sufficiently completed, for additional removal of carbon oxysulfides from the conversion product.
The rich tail gas conducted away from the bed during regeneration mode can be used as fuel, e.g., for fueling one or more fired heater, such as firing steam cracking furnaces and/or other combustion equipment. Because the sorbent bed is utilized in sorption mode for a relatively long time period, with bed regeneration occurring during a significantly shorter period (regeneration mode), the rich tail gas has a significantly greater content of carbon oxysulfides than does the “lean” tail gas conducted to the bed during regeneration mode. This undesirably increases the furnace's flue gas sulfur content. It is desired to develop processes which lessen or avoid this difficulty.